The line-start permanent magnet (LSPM) synchronous motor of the prior art is a hybrid motor with its stator structure being substantially the same as that of an AC induction motor or that of an AC synchronous motor. On the other hand, the rotor structure of this hybrid motor is a combination of the squirrel cage structure in the rotor of the AC induction motor and the permanent magnet structure in the rotor of the AC permanent magnet synchronous motor. When the stator of the LSPM synchronous motor is connected to the power source, a rotating magnetic field is generated by the stator, and an induction current is induced in the squirrel cage structure of the rotor structure. Consequently, the LSPM synchronous motor is started by a starting torque generated by the induction current until the rotating speed of the rotor becomes the same as that of the rotating magnetic field generated by the stator winding, i.e. the rotor reaches a synchronous speed, then, the induction current in the squirrel cage of the rotor vanishes. At this moment, the torque is not generated by the squirrel cage structure anymore but will thoroughly be generated by the interaction of the rotating magnetic field generated by the permanent magnet in the rotor and the stator's winding. Recently, due to the constantly improved material of the permanent magnet and “magnetic energy product”, the LSPM can attain a very high “operation efficiency”. However, the cogging torque of the permanent magnet synchronous motor of the prior art becomes very large since the permanent magnet with high magnetic energy product is used. As a result, the motor is subject to occurrence of operating vibration and noise. Generally, the stator provides skew channels to improve the demerit, but the winding work of the stator becomes rather difficult.
FIG. 1 is the U.S. Pat. No. 5,925,727, Boyd et al. of the prior art. As shown in FIG. 1, the permanent magnets 28 in the rotor 10 are disposed on the outer side of the squirrel cage conductive bar slot 26. Since the number of permanent magnets 28 used is as many as the number of the squirrel cage conductive bar slot 26, not only the assembling work of the permanent magnets 25 is rather complicated but the motor is subject to generate relatively large cogging torque.
FIG. 2 is the U.S. Pat. No. 5,097,166, Mikulic of the prior art. As shown in FIG. 2, the permanent magnet 64 in the rotor 30 is disposed on the inner side of the squirrel cage conductive bar slot 39. Since the rotor 30 is round in shape without special designed configuration, the cogging torque as well as the vibration and noise of the motor while the motor is in operation is still relatively large.
FIG. 3 is the U.S. Pat. No. 4,922,152, Gleghorn et al. of the prior art. As shown in FIG. 3, since the permanent magnet 18 between the two adjacent poles is disposed to extend up to the circumference of the rotor lamination□ thereby, the permanent magnet between the two poles has to place in the permanent magnet containing slot before the manufacturing process of casting the squirrel cage is performed. The reason is that the permanent magnet subjected to high temperature during the casting process can generate demagnetization.
FIG. 4 is the U.S. Pat. No. 4,748,359, Yahara et al. of the prior art. As shown in FIG. 4, the rotor can improve the cogging torque by adjusting the contour of the permanent magnet. However, since the rotor does not have squirrel cage structure provided, the rotor cannot be line-started without utilizing some other devices.
FIG. 5 is the U.S. Pat. No. 4,358,696, Liu et al. of the prior art. As shown in FIG. 5, two sets of symmetrically disposed permanent magnets constitute four poles. But since there are permanent magnets extended up to the circumference of the rotor lamination□ thereby, as described before, the permanent magnet between the two poles has to place in the permanent magnet containing slot before the manufacturing process of casting the squirrel cage is performed. The reason is that the permanent magnet subjected to high temperature during the casting process can generate demagnetization.
FIG. 6 is the U.S. Pat. No. 4,139,790, Steen et al. of the prior art. As shown in FIG. 6, the permanent magnet 27˜30 is disposed on the inner side of the permanent magnet apertures 23˜26 of the squirrel cage structure of the rotor 13. But, as described before, since the rotor 13 is round in shape without special designed configuration, therefore, the cogging torque as well as the vibration and noise of the motor while the motor is in operation are still relatively large.
FIG. 7 is the Taiwan patent 371,126, Kang et al. and FIG. 8 is the Taiwan patent 362,843, Kang et al. of the prior art. As shown in FIG. 7 and FIG. 8, the distance between the center of the rotor and the surface of the rotor varies with the radius of curvature of the surface of the rotor being smaller than that of a common rotor in round shape. However, just how the curvature of the surface of the rotor varies is not explained anywhere in their specifications. Moreover, in the Taiwan patent 371,126, the second permanent magnet containing slots 16 extends up to the circumference of the rotor lamination. Normally, the manufacturing process of the squirrel cage structure is to place the whole rotor lamination in the mold, then, the molten aluminum with melting point at 268° C. is poured into the conductive bar slots to form conductive bars. In addition, at both ends of the rotor lamination, end rings are formed by aluminum material to be connected to the ends of the rotor lamination and to each of the conductive bars to constitute an integrated squirrel cage structure. The technical features of the above-mentioned Taiwan patents TW 371,126 and TW 362,843 are that the permanent magnet containing slots extend up to the circumference of the rotor lamination, and the conductive bars containing slots of the squirrel cages of the rotor structure are disposed closed to the circumference of the rotor lamination too. Therefore, it is necessary to place the permanent magnets in position before the casting manufacturing process of the squirrel cage structure of the rotor performs. However, generally, among all the permanent magnets of rare earth metals having high magnetic energy products, the permanent demagnetization temperatures are less than 200° C. Placing the permanent magnets in the permanent magnet containing slots beforehand can cause the permanent magnets to be subject to demagnetization during the casting manufacturing process.
FIG. 9 is the Japan patent 2003-23740 (P2003-23740A) of the prior art. As shown in FIG. 9, a permanent magnet is disposed for each of the corresponding inner side of the magnetic poles. Moreover, the invention describes only that the arc of the surface of the rotor lamination for each poles is the connection between the location where the distance from the center of the rotor is maximum and the location where the distance from the center of the rotor is minimum. However, no explanation is given anywhere in the specification as to what kind of the curve of the arc it belongs to. What is more, the prior art does not provide any means to adequately attenuate the magnetic flux of the magnetic pole to reduce the self-retaining torque caused by the magnetic flux of the magnetic poles. Therefore, the prior art cannot improve the starting characteristic of the motor at the transient state during the operation from “stop” to “start” of the LSPM synchronous motor. Furthermore, as pointed out in the preceding discussion, since the rotor does not have squirrel cage structure provided, the rotor cannot be line-started without utilizing some other devices.
The foregoing statements are the disadvantages of the prior art.